This invention relates to blade outer air seals (BOAS) for gas turbine engines and, particularly to a variable BOAS arrangement that effectuates active clearance control.
In the gas turbine engine field of technology, a great effort has been devoted to improve engine operating performance by attempting to hold the clearance of the gap between the outer air seal and the tip of the turbine blade at a minimum during the full range of the engine's operating envelope. What has developed over the years is a host of structures and concepts that have through active and passive clearance controls attempted to achieve this end. To some degree, many of these structures and concepts have proven to be successful, but owing to the increasing demands on engine and aircraft performance, the problem has become increasingly more difficult. The solution to the problem is also predicated on the type of aircraft/engine being designed and to its particular mission. What may be satisfactory for say, aircraft used by commercial airlines is often not satisfactory for military aircraft, particularly those designed to be in the fighter class.
For example, in engines designed for use in aircraft used for commercial applications, such as those used in revenue service, the active clearance controls have been proven to be fairly successful. In one form, this type of control judiciously impinges air on the engine's outer case in proximity to the turbine rotor in order to shrink the case at predetermined times during its operating envelope and hence, position the outer air seal closer to the tips of the turbine or compressor blades so as to reduce the gap.
In contrast to the active clearance control design philosophy, the passive clearance control utilizes a continuous means to effectuate the control of these clearances. For example, one such system continuously impinges cooling air on the outer engine case in proximity to the rotor blades to limit the rate of expansion of the outer case subjected to elevated temperatures in order to hold clearances to a minimum. In short the “active” type of control requires a control system that responds to an input and applies hot or cold air or mechanical means in order to effectuate control of the clearances. The “passive” type of clearance control doesn't require a control system and is in a quiescent state at all times.
Historically, it has been observed that high pressure turbine blades wear in to the respective BOAS in a variable manner. The clearance control systems currently utilized, either passive or active, are not capable of moving the BOAS away from the blades and maintaining clearance at a high rate or across the entire engine envelope. This significantly affects performance of engines, especially newer commercial engines.